Audio microphones are commonly used in a variety of consumer applications such as cellular telephones, digital audio recorders, personal computers and teleconferencing systems. In particular, lower-cost electret condenser microphones (ECM) are used in mass produced cost sensitive applications. An ECM microphone typically includes a film of electret material that is mounted in a small package having a sound port and electrical output terminals. The electret material is adhered to a diaphragm or makes up the diaphragm itself. Most ECM microphones also include a preamplifier that can be interfaced to an audio front-end amplifier within a target application such as a cell phone. The output of the front-end amplifier can be coupled to further analog circuitry or to an A/D converter for digital processing. Because an ECM microphone is made out of discrete parts, the manufacturing process involves multiple steps within a complex manufacturing process. Consequently, a high yielding, low-cost ECM microphone that produces a high level of sound quality is difficult to achieve.
In a microelectro-mechanical Systems (MEMS) microphone, a pressure sensitive diaphragm is etched directly onto an integrated circuit. As such, the microphone is contained on a single integrated circuit rather than being fabricated from individual discrete parts. The monolithic nature of the MEMS microphone produces a higher yielding, lower cost microphone.
The interfacing of a MEMS microphone or sensor with an electrical system, however, poses a number of difficulties because of the microphone's very high output impedance. For example, loading by the preamplifier can potentially attenuate the microphone's output signal, and the high resistance nature of the MEMS microphone makes it prone to EMI disturbance and power supply disturbances due to a poor power supply rejection ratio (PSRR).